1. Field of the Invention
The present invention relates to a focus detection apparatus which receives the light transmitted through an objective lens and detects the focus condition of the objective lens.
2. DESCRIPTION OF THE PRIOR ART
Various types of focus detection apparatus have been known for automatic focusing. FIG. 1 shows a type of a basic optical system of a focus detection apparatus which is related to the present invention, wherein an imageseparating optical system is used. A condenser lens L.sub.0 is set close to or behind a predetermined focal plane F of an objective lens TL, and a pair of image-re-forming lenses L.sub.1 and L.sub.2 are set behind the condenser lens L.sub.0 symmetrically with respect to the optical axis X of the objective lens TL, with their optical axes parallel to the optic axis X. A light-limiting mask (stop) M is set before the image-reforming lenses L.sub.1, L.sub.2.
In the optical system, the focus condition of the objective lens TL can be detected by comparing two images formed on a predetermined focal plane FR of the image-reforming lenses L.sub.0, L.sub.1, as will be explained below. If the objective lens TL is brought into an in-focus condition for an object, an image A of the object is formed on the predetermined focal plane F, and a first image A.sub.1 and a second image A.sub.2 of the image A are formed by the image-reforming lenses L.sub.1 and L.sub.2, respectively, on the predetermined focal plane FR. On the other hand, if the objective lens TL is in a rear-focus condition, an image B of the object is formed behind the image A, and a first image B.sub.1 and a second image B.sub.2 of the image B are formed by the image-re-forming lenses L.sub.1 and L.sub.2 at a position farther than that of each of the first image A.sub.1 and the second image A.sub.2 in a direction perpendicular to the optical axis, respectively. On the contrary, if the objective lens TL is in a front-focus condition, an image C of the object is formed before the image A, and first and second images C.sub.1 and C.sub.2 of the image C are formed by the image-re-forming lenses L.sub.1 and L.sub.2 at a nearer position to the optic axis X, respectively. If a pair of photoelectric converters (not shown) are arranged on the image-re-forming plane FR, the positions of the first and the second images formed by the image-re-forming lenses L.sub.1 and L.sub.2 can be detected so that it can be decided whether the image of an object is in focus or out of focus.
FIG. 2 shows a relation between the pupil of the objective lens and the exit pupil of the optical system. The condenser lens L.sub.0 operates for projecting the images of the pupils of the image-re-forming lenses L.sub.1, L.sub.2 toward the objective lens side, and the light-limiting mask M which has two apertures for separation is arranged before the imagere-forming lenses L.sub.1, L.sub.2, and the image of the mask M is projected on a position b in the front side of the condenser lens L.sub.0.
A cone defined by lines FN indicates a width of light bundle for obtaining an identical brightness on the plane F. However, if the exit pupil of the objective lens TL is set behind the position b, for example at a position c, a beam along a line l.sub.1 does not reach to the plane F, and the brightness at end portions Q.sub.2 and P.sub.1 of images on the plane FR are decreased while the F-number is kept constant. On the contrary, if the exit pupil of the objective lens TL is set before the position b, for example at a position a, a beam along a line u.sub.1 does not reach to the plane F, and the brightness at end portions Q.sub.1 and P.sub.2 of the images on the plane FR are decreased. If the pupils of the image-re-forming lenses L.sub.1, L.sub.2 are shaded as mentioned above, errors might be caused upon detecting the focus conditions. Therefore, the light bundle transmitted through the objective lens can be used most efficiently for the focus detection in the case that the position of the exit pupil of the objective lens TL coincides with the position b of the projected image-formed lenses L.sub.1 and L.sub.2.
A conventional focus detection apparatus as explained above has been used mainly for an objective lens system of dioptric type. However, it is impossible to use the focus detection apparatus of this type for a catadioptric lens system such as shown by way of an example in FIG. 3. It is desirable that an automatic focus detection apparatus can be used also for a cata-dioptric lens system because it has following advantages:
(a) A cata-dioptric lens system consists of a fewer number of simple lenses, and it is cheaper than a lens of refraction type.
(b) Its total length can be shortened by making use of reflection and its weight can be lessened.
(c) The influence of color dispersion of glass is small owing to reflection, and the chromatic aberration is much smaller than that of a lens system of refraction type.
As for a cata-dioptric lens system, the light does not enter through the central portion of the exit pupil thereof. As shown in FIG. 4, since the light close to an optic axis X of the cata-dioptric lens system is shaded, the configuration of the pupil thereof becomes an annular area defined between an outer circle U and an inner circle L. Thus, the bad effect due to shading on focus detection increases. Therefore, it becomes much more desirable to coincide the projected positions of image-re-forming lenses with the position of the exit pupil of the lens system.
Further a concave lens is used as a rearmost lens element in order to shorten the total length of the catadioptric lens system so that the exit pupil thereof locates at a somewhat farther position than that of a lens of refraction type of the same focal length, and the F-number thereof becomes relatively large. From this point of view the coincidence of the projected positions of image-reforming lenses with that of the exit pupil also becomes more desirable.
FIGS. 5-7 show examples of shapes of pupils E' of prior art image-re-forming lenses projected on the exit pupil of objective lens. A pupil E' exists inside the out line U' of the exit pupil of objective lens. Prior art pupils E' shown in FIG. 5 (see Japanese Patent Laid Open Publication No. 32013/1985) are substantially oval; an outer side of the pupil E' has a curvature similar to the outline U' while the other (inner side) has a curvature substantially symmetrical to said outer side curvature. A prior art pupil E' shown in FIG. 6 (see Japanese Patent Laid Open Publication No. 130725/1981) has an outer side of a straight lien and an inner side of a straight line, both being parallel to each other; it also has an upper and a lower side each being an arc; the length of the pupil in the longitudinal direction thereof is made longer than the width in the transversal direction. A prior art pupil E' shown in FIG. 7 (see U.S. Pat. No. 4,370,551) has an outer side periphery which extends parallel to the outline U' of the pupil of the objective lens, while it has an inner side periphery being a straight line. These pupils E' as shown in FIGS. 5-7 are not proper to a cata-dioptric lens system since nothing is considered with respect to a possible interference between the inner periphery L of the pupil of the cata-dioptric lens system (see FIG. 4) and the inner side portion of each pupil E' of the image-re-forming lenses.